The present invention relates generally to high-impedance surfaces. More particularly, the present invention relates to a multi-resonant, high-impedance electromagnetic surface.
A high impedance surface is a lossless, reactive surface whose equivalent surface impedance,             Z      s        =                  E        tan                    H        tan              ,
approximates an open circuit and which inhibits the flow of equivalent tangential electric surface current, thereby approximating a zero tangential magnetic field, Htan≈0. Etan and Htan are the electric and magnetic fields, respectively, tangential to the surface. High impedance surfaces have been used in various antenna applications. These applications range from corrugated horns which are specially designed to offer equal E and H plane half power beamwidths to traveling wave antennas in planar or cylindrical form. However, in these applications, the corrugations or troughs are made of metal where the depth of the corrugations is one quarter of a free space wavelength, xcex/4, where xcex is the wavelength at the frequency of interest. At high microwave frequencies, xcex/4 is a small dimension, but at ultra-high frequencies (UHF, 300 MHz to 1 GHz), or even at low microwave frequencies (1-3 GHz), xcex/4 can be quite large. For antenna applications in these frequency ranges, an electrically-thin (xcex/100 to xcex/50 thick) and physically thin high impedance surface is desired.
One example of a thin high-impedance surface is disclosed in D. Sievenpiper, xe2x80x9cHigh-impedance electromagnetic surfaces,xe2x80x9d Ph.D. dissertation, UCLA electrical engineering department, filed January 1999, and in PCT Patent Application number PCT/US99/06884. This high impedance surface 100 is shown in FIG. 1. The high-impedance surface 100 includes a lower permittivity spacer layer 104 and a capacitive frequency selective surface (FSS) 102 formed on a metal backplane 106. Metal vias 108 extend through the spacer layer 104, and connect the metal backplane to the metal patches of the FSS layer. The thickness h of the high impedance surface 100 is much less than xcex/4 at resonance, and typically on the order of xcex/50, as indicated in FIG. 1.
The FSS 102 of the prior art high impedance surface 100 is a periodic array of metal patches 110 which are edge coupled to form an effective sheet capacitance. This is referred to as a capacitive frequency selective surface (FSS). Each metal patch 110 defines a unit cell which extends through the thickness of the high impedance surface 100. Each patch 110 is connected to the metal backplane 106, which forms a ground plane, by means of a metal via 108, which can be plated through holes. The periodic array of metal vias 108 has been known in the prior art as a rodded media, so these vias are sometimes referred to as rods or posts. The spacer layer 104 through which the vias 108 pass is a relatively low permittivity dielectric typical of many printed circuit board substrates. The spacer layer 104 is the region occupied by the vias 108 and the low permittivity dielectric. The spacer layer is typically 10 to 100 times thicker than the FSS layer 102. Also, the dimensions of a unit cell in the prior art high-impedance surface are much smaller than xcex at the fundamental resonance. The period is typically between xcex/40 and xcex/12.
A frequency selective surface is a two-dimensional array of periodically arranged elements which may be etched on, or embedded within, one or multiple layers of dielectric laminates. Such elements may be either conductive dipoles, patches, loops, or even slots. As a thin periodic structure, it is often referred to as a periodic surface.
Frequency selective surfaces have historically found applications in out-of-band radar cross section reduction for antennas on military airborne and naval platforms. Frequency selective surfaces are also used as dichroic subreflectors in dual-band Cassegrain reflector antenna systems. In this application, the subreflector is transparent at frequency band f1 and opaque or reflective at frequency band f2. This allows one to place the feed horn for band f1 at the focal point for the main reflector, and another feed horn operating at f2 at the Cassegrain focal point. One can achieve a significant weight and volume savings over using two conventional reflector antennas, which is critical for space-based platforms.
The prior art high-impedance surface 100 provides many advantages. The surface is constructed with relatively inexpensive printed circuit technology and can be made much lighter than a corrugated metal waveguide, which is typically machined from a block of aluminum. In printed circuit form, the prior art high-impedance surface can be 10 to 100 times less expensive for the same frequency of operation. Furthermore, the prior art surface offers a high surface impedance for both x and y components of tangential electric field, which is not possible with a corrugated waveguide. Corrugated waveguides offer a high surface impedance for one polarization of electric field only. According to the coordinate convention used herein, a surface lies in the xy plane and the z-axis is normal or perpendicular to the surface. Further, the prior art high-impedance surface provides a substantial advantage in its height reduction over a corrugated metal waveguide, and may be less than one-tenth the thickness of an air-filled corrugated metal waveguide.
A high-impedance surface is important because it offers a boundary condition which permits wire antennas conducting electric currents to be well matched and to radiate efficiently when the wires are placed in very close proximity to this surface (e.g., less than xcex/100 away). The opposite is true if the same wire antenna is placed very close to a metal or perfect electric conductor (PEC) surface. The wire antenna/PEC surface combination will not radiate efficiently due to a very severe impedance mismatch. The radiation pattern from the antenna on a high-impedance surface is confined to the upper half space, and the performance is unaffected even if the high-impedance surface is placed on top of another metal surface. Accordingly, an electrically-thin, efficient antenna is very appealing for countless wireless devices and skin-embedded antenna applications.
FIG. 2 illustrates electrical properties of the prior art high-impedance surface. FIG. 2(a) illustrates a plane wave normally incident upon the prior art high-impedance surface 100. Let the reflection coefficient referenced to the surface be denoted by xcex93. The physical structure shown in FIG. 2(a) has an equivalent transverse electromagnetic mode transmission line shown in FIG. 2(b). The capacitive FSS 102 (FIG. 1) is modeled as a shunt capacitance C and the spacer layer 104 is modeled as a transmission line of length h which is terminated in a short circuit corresponding to the backplane 106. FIG. 2(c) shows a Smith chart in which the short is transformed into the stub impedance Zstub just below the FSS layer 102. The admittance of this stub line is added to the capacitive susceptance to create a high impedance Zin at the outer surface. Note that the Zin locus on the Smith Chart in FIG. 2(c) will always be found on the unit circle since our model is ideal and lossless. So xcex93 has an amplitude of unity.
The reflection coefficient xcex93 has a phase angle xcex8 which sweeps from 180xc2x0 at DC, through 0xc2x0 at the center of the high impedance band, and rotates into negative angles at higher frequencies where it becomes asymptotic to xe2x88x92180xc2x0. This is illustrated in FIG. 2(d). Resonance is defined as that frequency corresponding to 0xc2x0 reflection phase. Herein, the reflection phase bandwidth is defined as that bandwidth between the frequencies corresponding to the +90xc2x0 and xe2x88x9290xc2x0 phases. This reflection phase bandwidth also corresponds to the range of frequencies where the magnitude of the surface reactance exceeds the impedance of free space: |X|xe2x89xa7xcex70=377 ohms.
A perfect magnetic conductor (PMC) is a mathematical boundary condition whereby the tangential magnetic field on this boundary is forced to be zero. It is the electromagnetic dual to a perfect electric conductor (PEC) upon which the tangential electric field is defined to be zero. A PMC can be used as a mathematical tool to create simpler but equivalent electromagnetic problems for slot antenna analysis. PMCs do not exist except as mathematical artifacts. However, the prior art high-impedance surface is a good approximation to a PMC over a limited band of frequencies defined by the +/xe2x88x9290xc2x0 reflection phase bandwidth. So in recognition of its limited frequency bandwidth, the prior art high-impedance surface is referred to herein as an example of an artificial magnetic conductor, or AMC.
The prior art high-impedance surface offers reflection phase resonances at a fundamental frequency, plus higher frequencies approximated by the condition where the electrical thickness of the spacer layer, xcex2h, in the high-impedance surface 100 is nxcfx80, where n is an integer. These higher frequency resonances are harmonically related and hence uncontrollable. If the prior art AMC is to be used in a dual-band antenna application where the center frequencies are separated by a frequency range of, say 1.5:1, we would be forced to make a very thick AMC. Assuming a non-magnetic spacer layer (xcexcD=1), the thickness h must be h=xcex/14 to achieve at least a 50% fractional frequency bandwidth where both center frequencies would be contained in the reflection phase bandwidth. Alternatively, magnetic materials could be used to load the spacer layer, but this is a topic of ongoing research and nontrivial expense. Accordingly, there is a need for a class of AMCs which exhibit multiple reflection phase resonances, or multi-band performance, that are not harmonically related, but at frequencies which may be prescribed.
By way of introduction only, in a first aspect, an artificial magnetic conductor includes a frequency selective surface having a frequency dependent permeability xcexc1z in a direction normal to the frequency dependent surface, a conductive ground plane, and a rodded medium disposed between the frequency selective surface and the conductive ground plane.
In another aspect, an artificial magnetic conductor includes a conductive ground plane and a spacer layer disposed on the ground plane. One or more arrays of coplanar loops are resonant at two or more frequency bands, each loop having a similar shape and similar size. The one or more arrays of coplanar loops produce a frequency dependent normal permeability xcexcz.
In another aspect, a disclosed electrical apparatus includes a conductive ground plane and a dielectric layer perforated by conductive rods in electrical contact with the conductive ground plane. The electrical apparatus further includes a frequency selective surface (FSS) disposed on the dielectric layer. The FSS includes a first layer of capacitively coupled loops resonant at a first frequency, a dielectric spacer layer and a second layer of capacitively coupled loops resonant at a second frequency. The frequency selective surface has a frequency dependent permeability in a direction substantially normal to the frequency selectively surface.
The foregoing summary has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.